Atomic spectroscopy is a technique useful for analyzing the constituent elements of a sample of material of interest. By way of example, the sample may be processed in an ultrasonic processor for the determination of metallic and semimetallic elements in the sample, using flame atomic absorption or graphite furnace atomic absorption. A typical sample to be subjected to such elemental analysis consists of a slurry of insoluble matter in a liquid suspension, such as solid particles suspended in solution.
Of particular interest is the elemental analysis of slurries in which solid particles of 325 mesh (44 microns) sieve designation or smaller, and more particularly in the range from 50 to 170 mesh (297 to 88 microns) sieve designation, are suspended in water or an organic liquid such as methanol. A consistent slurry, in terms of regular particulate size and concentration in the solution, tends to produce a more precise and accurate analysis than would otherwise be achieved. Yet, it is difficult to obtain the desired precision and accuracy in the elemental analysis of a slurry. This is because the slurry is formed by mixing its constituents together, after which a portion of the slurry is taken as a sample and transferred to a sample cup for purposes of performing the analysis. With the passage of even a brief period of time from the mixing to the transfer and commencement of analysis, the solid particles have begun to precipitate or settle out of the suspension. As a consequence, the true elemental content of the slurry may escape analysis. It is imperative that the sample be kept in uniform suspension to assure accurate analysis and determination of the elements of interest.
As a result, it would be desirable to provide a method and apparatus for improving the accuracy and precision of spectroscopic analysis of a slurry sample by maintaining the solid particles of the sample in suspension by agitation thereof until virtually the last instant prior to performing the analysis.
Prior art techniques for agitating liquid samples prior to analysis generally involve the use of special means for shaking the apparatus and holding the sample, such as a vibrating table or receptacle. A representative example of such prior art techniques is described in U.S. Pat. No. 4,265,544 to Banno et al (hereinafter referred to as the "Banno technique"). The Banno technique is intended for automatic agitation of a reaction solution consisting of a mixture of a liquid to be examined and a reagent. In its implementation, mechanical vibration is imparted to apparatus holding the sample by a rotary solenoid energized for a preselected time interval by an oscillator. During that interval, the sample holder undergoes reciprocating movement.
Such a technique requires that the entire apparatus cease moving before the analysis can be performed, or that the sample be transferred to another location for analysis. Because the sample undergoing analysis in the Banno technique is not a slurry, but a solution composed entirely of a liquid and a reagent, the effect of a delay between agitation and analysis may be minor in that circumstance. That is, the settling down of the apparatus is not likely to be accompanied by a settling out of the particulate matter from solution. Rather, the Banno technique addresses the problem of agitating the solution in a manner that will keep its constituents thoroughly mixed while avoiding a change of the temperature of the sample which might affect the analysis.
Prior art techniques for agitating a liquid sample under analysis typically present the further difficulty of requiring the movement of a large mass, such as the reciprocation of a holding table, in order to achieve the desired agitation. In such arrangements, it is often necessary to provide complex and bulky apparatus to produce the movement of the larger mass, and thereafter dampen the movement to return the mass to a steady state within a reasonably brief period of time, so that the analysis may be commenced while the solution is still thoroughly mixed.
It has been proposed in the prior art to agitate a sample under analysis by manually inserting an ultrasonic probe into the sample cup, and thereby keep the solid particles in suspension prior to performing the analysis. This technique, however, interferes with the smooth continuous operation of the atomic spectroscopy apparatus, as it requires that the apparatus be stopped during the agitation step. Moreover, the manual step must be performed with great care to avoid disturbing any portion of the apparatus or of the sample cups themselves. Also, manual activation and deactivation of the probe, coupled with the need to restart the equipment thereafter, allows at least partial resettlement of particles from the suspension, thereby making the results of the analysis less accurate.
It is another object of the present invention to provide methods and apparatus for agitating a slurry under analysis in a manner which allows the analysis to be performed virtually instantaneously after the agitation has ceased, without the need to wait for the apparatus to settle down.
Yet another object of the present invention is to provide a technique for maintaining solid particles in liquid suspension in a slurry sample under spectroscopic analysis to determine the elemental content thereof, by means which avoid the need for movement of massive parts of an apparatus.
It is yet another object of the present invention to provide an improved technique for maintaining the solid particles of a slurry sample under analysis suspended in solution automatically, that is, by a self-acting mechanism that repeatedly agitates the slurry, without interfering with the apparatus performing the analysis.